1. Field of the Invention
This present invention relates to a hydraulic machine adapted to serve either as a hydraulic turbine or a pump which includes a runner having formed therein a plurality of balance holes adapted to maintain the hydraulic pressure thrust constant at all times.
2. Description of the Prior Art
A runner of the Francis type of the prior art which is used with a hydraulic machine generally comprises a plurality of blades interposed between two rings or a crown ring and a shround ring, and pressure chambers adapted to be filled with pressure water are formed above and below the runner, respectively.
More specifically, the upper surface of the crown ring and an upper cover disposed in spaced juxtaposed relation therewith define therebetween the pressure chamber that is a runner crown chamber while the underside of the shround ring and a lower cover define therebetween the pressure chamber that is a runner band chamber.
Pressure water flows into the runner crown chamber and runner band chamber to press and move the runner upwardly and downwardly, with the differential pressure of the pressure water manifesting itself as an axial thrust applied to the runner of the hydraulic machine. Generally, the crown ring has a greater pressure receiving area than the shround ring, so that the thrust acts downwardly. To bear this thrust, a rotary shaft directly connected to the runner is supported by a thrust bearing.
In order to economically obtain a compact size in a thrust bearing, attempts have been made to reduce the downwardly directed thrust by radially dividing the runner crown chamber by an intermediate seal into an inner runner crown chamber and an outer runner crown chamber.
The intermediate seal is usually of the same diameter as an inner peripheral seal provided in the runner band chamber. With this arrangement, the downwardly directed thrust applied by the outer runner crown chamber and the upwardly directed thrust applied by the runner band chamber cancel each other out. A plurality of balance holes each have one end which opens at the upper surface of the crown ring which cooperates with the upper cover to form the inner runner crown chamber, so that the pressure in the inner runner crown chamber can be released to a low pressure section in the runner through the balance holes.
That is, the plurality of balance holes are formed in the runner to maintain communication therethrough between the inner runner crown chamber and the inner side of a runner boss or the inner side of the joint between the under side of the crown ring and each of the blades. This serves to lower the pressure in the inner runner crown chamber so that the lowered pressure can function as a downwardly directed thrust.
However, with such balance holes, there has been little effect in reducing the downwardly directed thrust, because, as indicated by a curve B in FIG. 2, the pressure in the inner and outer runner crown chamber rises so as to increase the downwardly directed thrust as the number of revolutions of the runner increases. This is considered due to the fact that, assuming that the number of revolutions of the runner is zero and the water flows inwardly from the outer periphery of the runner toward the inside thereof, the water is guided by the blades which have a suitable curvature radially of the runner, so that the water will be caused to flow in a swirling stream in the center of the runner or below the cover of the runner boss. This swirling stream of water produces a low pressure section in the interior of the runner boss. Therefore, the pressure in the runner crown chamber is lowered through the balance holes which open at the inside of the cover of the runner boss only when the number of revolutions is zero or near the zero point as shown by the characteristic curve B in FIG. 2. Also, the orientation or inclination of the balance holes is considered to be part of the cause of the increase in the runner crown chamber pressure, as water in the balance holes is moved outwardly or toward openings in the runner crown chamber by centrifugal forces as the number of revolutions of the runner increases.
Accordingly, in the prior art, it has been necessary to design and manufacture the thrust bearing by taking into consideration the rise of the hydraulic thrust despite of the provisions of the balance holes.